At 8MHz, therefore a 2MHz instruction clock that presetting 50 for TMR1L to match against would - if I have the maths/theory correct - mean a a 40khz special event trigger rate......but if that were the case, I'd have expected to seen the ADC samples be ing far more erratic at an audio input signal 20khz than what I'm seeing (two samples during the 20Khz waveform would have ADC results all over the place) ...so what's going on?
I believe you are correct in the math, maybe you are off by a count of 1 for reload or something. The more perplexing part to me is the scope showing 30 Hz!! That really confuses me.

Is the incoming signal comming from a signal generator? if so it is prolly a really nice clean 20K? So if this is the case, it stands to reason the samples are being taken at the same spot maybe? since 20k is half the sample frequency.

What if the signal is 19.367K? (random number)